Dilysine-Methylene Diphenyl Diisocyanate (MDI), a Urine Biomarker of MDI Exposure?

Lung Gene Expression Suggests Roles for Interferon-Stimulated Genes and Adenosine Deaminase Acting against RNA-1 in Pathologic Responses to Diisocyanate

Mechanisms underlying methylene diphenyl diisocyanate (MDI) and other low molecular weight chemical-induced asthma are unclear and appear distinct from those of high molecular weight (HMW) allergen-induced asthma. We sought to elucidate molecular pathways that differentiate asthma-like pathogenic vs nonpathogenic responses to respiratory tract MDI exposure in a murine model. Lung gene expression differences in MDI exposed immune-sensitized and nonsensitized mice vs unexposed controls were measured by microarrays, and associated molecular pathways were identified through bioinformatic analyses and further compared with published studies of a prototypic HMW asthmagen (ovalbumin). Respiratory tract MDI exposure significantly altered lung gene expression in both nonsensitized and immune-sensitized mice, vs controls. Fifty-three gene transcripts were altered in all MDI exposed lung tissue vs controls, with levels up to 10-fold higher in immune-sensitized vs nonsensitized mice. Gene transcripts selectively increased in MDI exposed immune-sensitized animals were dominated by chitinases and chemokines and showed substantial overlap with those increased in ovalbumin-induced asthma. In contrast, MDI exposure of nonsensitized mice increased type I interferon stimulated genes (ISGs) in a pattern reflecting deficiency in adenosine deaminase acting against RNA (ADAR-1), an important regulator of innate, as well as "sterile" or autoimmunity triggered by tissue damage. Thus, MDI-induced changes in lung gene expression were identified that differentiate nonpathogenic innate responses in nonsensitized hosts from pathologic adaptive responses in immune-sensitized hosts. The data suggest that MDI alters unique biological pathways involving ISGs and ADAR-1, potentially explaining its unique immunogenicity/allergenicity.

Methylenediphenyl diisocyanate lysine conjugates in the urine of workers exposed to methylenediphenyl diisocyanate

Diisocyanates have long been a leading cause of occupational asthma. As control often relies on personal protective equipment and there is the potential for skin uptake, biological monitoring is often used to assess worker exposure. Current routine biological monitoring methods do not distinguish between a diisocyanate and the corresponding diamine exposure in urine samples; therefore, a specific urinary biomarker is desirable. Urine samples were obtained from a group of workers exposed to methylenediphenyl diisocyanate (MDI) where aerosol generation was unlikely. Lysine conjugates of MDI were extracted from urine by solid phase extraction; analysis was performed by liquid chromatography tandem mass spectrometry. Acetylated MDI-lysine (acMDI-Lys) conjugates were detected in 73% of samples tested from persons with exposure to MDI compared to 93% of samples that were positive for methylene dianiline (MDA) in hydrolysed urine. There was a weak but significant positive correlation between the two biomarkers (r² = 0.377). This is the first report detecting and quantifying acMDI-Lys in the urine of workers exposed to MDI, and acMDI-Lys may be a useful non-invasive biomarker in discriminating between MDI and MDA exposures.

Absorption, distribution, metabolism, and excretion of methylene diphenyl diisocyanate and toluene diisocyanate: Many similarities and few differences

Methylene diphenyl diisocyanate (MDI) and toluene diisocyanate (TDI) are high production volume chemicals used for the manufacture of polyurethanes. For both substances, the most relevant adverse health effects after overexposure in the workplace are isocyanate-induced asthma, lung function decrement and, to a much lesser extent, skin effects. Over the last two decades many articles have addressed the reactivity of MDI and TDI in biological media and the associated biochemistry, which increased the understanding of their biochemical and physiological behavior. In this review, these new insights with respect to similarities and differences concerning the adsorption, distribution, metabolism, and excretion (ADME) of these two diisocyanates and the implications on their toxicities are summarized. Both TDI and MDI show very similar behavior in reactivity to biological macromolecules, distribution, metabolism, and excretion. Evidence suggests that the isocyanate (NCO) group is scavenged at the portal-of-entry and is not systemically available in unbound reactive form. This explains the lack of other than portal-of-entry toxicity observed in repeated-dose inhalation tests.

Molecular Characterization and Experimental Utility of Monoclonal Antibodies with Specificity for Aliphatic Di- and Polyisocyanates

Aliphatic di- and polyisocyanates are crucial chemical ingredients in many industrial processes and are a well-recognized cause of occupational asthma. Serologic detection of "chemical epitopes" in biological samples could serve as an exposure surveillance approach toward disease prevention, and thus we sought to generate aliphatic isocyanate-specific monoclonal antibodies (mAbs). Three hybridomas were generated from Balb/c mice immunized with a commercial product containing a combination of uretdione, homopolymer, and monomeric forms of hexamethylene diisocyanate (HDI). Three stable hybridomas were subcloned by limiting dilution, two secreting IgG1κ and one secreting IgMκ mAb that bind aliphatic di- and polyisocyanates (conjugated to albumin), but not aromatic toluene or methylene diphenyl diisocyanate (TDI or MDI). Each mAb demonstrates slight differences in epitope specificity, for example, recognition of hydrogenated MDI (HMDI) or different carrier proteins (transferrin, actin) reacted with vapor phase HDI, and is encoded by unique recombination of different germline antibody genes, with distinct complementary determining regions. By western blot, all three mAbs detect a molecule with characteristics of an albumin adduct uniquely in urine from mice skin exposed to a mixture of aliphatic di- and polyisocyanate. Together, the data define molecular determinants of humoral immune recognition of aliphatic di- and polyisocyanates through new mAbs, which will serve as useful research reagents and may be applicable to future exposure surveillance efforts.

Assessment and control of exposures to polymeric methylene diphenyl diisocyanate (pMDI) in spray polyurethane foam applicators

In this work we characterize personal inhalation and dermal exposures to diphenyl methane diisocyanate (MDI) and other species in polymeric MDI (pMDI) formulations during spray polyurethane foam (SPF) insulation at 14 sites in New England. We further assess the adequacy of current workplace practices and exposure controls via comparative urinary biomonitoring of the corresponding methylene diphenyl diamine (MDA) pre- and post-shift. MDI and pMDI are potent dermal and respiratory sensitizers and asthmagens, strong irritants of the skin, eyes, and the respiratory tract, and may cause skin burns. This study is the first comprehensive report to-date on the work practices, inhalation and dermal exposures to isocyanates and effectiveness of existing controls during SPF applications. Breathing zone exposures to 4,4' MDI (n = 31; 24 sprayers, 7 helpers) ranged from 0.9 to 123.0 μg/m3 and had a geometric mean (GM) of 13.8 μg/m3 and geometric standard deviation (GSD) of 4.8. Stationary near field area samples (n = 15) were higher than personal exposures: GM, 40.9 (GSD, 3.9) μg/m3, range 1.4-240.8 μg/m3. Sixteen percent of personal air samples and 35% of area samples exceeded the National Institute for Occupational Health and Safety's (NIOSH) full shift recommended exposure limit (REL) of 50 μg/m3, assuming zero exposure for the unsampled time. 4,4' MDI load on the glove dosimeters had a GM of 11.4 (GSD 2.9) μg/glove pair/min, suggesting high potential for dermal exposures. Urinary MDA had a GM of 0.7 (GSD, 3.0) μmol MDA/mol creatinine (range, nd-14.5 μmol MDA/mol creatinine). Twenty-five % of urine samples exceeded the Health and Safety Executive (HSE) biological monitoring guidance value (BMGV) of 1 μmol MDA/mol creatinine. We further report on field observations regarding current exposure controls, discuss implications of these findings and opportunities for improving work practices to prevent isocyanate exposures during SPF insulation.